Presently, phosphate rock is mined by stripping the overburden from the underlying phosphate bearing matrix, slurrying the matrix, consisting of a mixture of clay, sand and phosphate pebble, with water and pumping the slurry from the mine site to a beneficiation plant where the matrix is washed, scrubbed and beneficiated to produce an upgraded pebble, phosphate rock concentrates, a slimes slurry and sand tailings. The slimes slurry consists mainly of clays, sand and very fine particles of phosphate rock. Generally, about 0.75 to 1 part by weight of slimes slurry is produced per part by weight of upgraded pebble and rock concentrate. In present practice, the simes, a dilute aqueous suspension of about 2 to 5 weight percent solids containing about 30 to 40 weight percent of the phosphate matrix, are disposed of by pumping into settling ponds built over previously mined areas. Over a period of time, the entrained solids slowly settle.
As presently practiced, slimes disposal procedures have many clearly defined and long recognized deficiencies. Among them is the need for large land areas set aside for slimes ponds. For example, a plant producing 2 million tons of phosphate rock per year will, over a period of 15 years, require approximately 4,500 acres of land for slimes ponds. And because of the slow settling rate of the entrained solids, such land is unusable for decades. Further, since the volume of the hydrated slimes and overburden is approximately 1.5 times greater than the volume of the mine pits, unsightly retaining dams often 35 feet high are necessary around the perimeter of each disposal area. The possibility of dam breakage always exists with resulting river, lake and stream pollution. Also of great importance is the use and retention of large quantities of water.
Another important deficiency in the percent procedures is the poor recovery of phosphate values from the matrix since the 30 to 40% of the phosphate values retained in the slimes are never recovered.
The recognition of these deficiencies has led experimenters to propose various alternatives over the years. Flotation techniques for ultra-fine particles to separate clays and phosphate values have been unsuccessful in terms of both cost and recovery.
Acid leaching of slimes has not been practical due to extreme difficulty in filtering the clays and other gangue materials from the slurries and the relatively low concentration of phosphate in the filtrate. Further, since the clays and other gangue in both matrix and slimes contain relatively high levels of acid-soluble iron, aluminum and magnesium compounds compared to beneficiated rock, acid leaching of such materials results in phosphoric acid of inferior quality.
Other methods of slimes disposal have been proposed, including using the slimes as aggregates for highway construction or as bricks for home construction or thickening the slimes to provide for improved water conservation and more rapid land reclamation. These methods have not proved practical.
One other method has been proposed to remove the slimes disposal problem; the use of dry unbeneficiated matrix. Previous attempts in this area have failed because of process difficulties and the poor quality of the phosphoric acid produced. These problems arise from the fact that the presence of a significant amount of clay results in poor filtration and from the further fact that the high iron and aluminum content of the clays affects calcium sulfate crystal size and form and product purity.
Thus, there presently exists a need for a slimes disposal process which will accomplish practicably the following design objectives: (1) recovery of presently discarded phosphate values; (2) elimination of the pond system for slimes disposal; (3) conservation of process water; and (4) provision of potentially closed loop operation of the phosphate mining and phosphoric acid production facility.